The present invention relates to a camera having an auto-focus function, and more particularly, the present invention relates to a camera having an auto-focus function that prevents out-of-focus images resulting from changes in object distance during an exposure process or operation.
An auto-focusing function of a camera automatically focuses a photographic subject to prevent a photographic image taken by a photographer from being out-of-focus, without requiring the photographer to have special training. Current cameras that include an auto-focusing function are equipped with an auto-focus ("AF") mirror provided in an AF drive of a focusing lens group, an AF sensor for detecting an in-focus state, and a photographic optical system. The AF mirror introduces a portion of the photographic light into the AF sensor which is typically a CCD element, for example. The AF sensor determines whether an in-focus state is achieved by detecting lateral displacement of the photographic image formed by the light from the AF mirror. The AF drive moves a focusing lens to a position that corresponds to the in-focus state based on an output signal from the AF sensor.
The above noted series of operations effected by the AF sensor for reaching an in-focus state are executed during a photographic preparation mode when the photographer partially presses a shutter release button. If the shutter release button is pressed to a fully pressed position after the in-focus state is achieved by the AF sensor, a photographic operation takes place. During the photographic operation, the AF mirror is raised to recede outside of the photographic optical system, enabling photographic light to reach a photographic plane. Subsequently, an exposure occurs while the shutter is opened for a specified period of time.
Since the AF mirror recedes outside of the photographic optical system, the in-focus state cannot be confirmed during the photographic operation. In addition, since the photographic light is not introduced to the AF sensor during an exposure process of the photographic operation, the in-focus state of the photographic optical system cannot be confirmed during the exposure process. As a result, during the photographic operation of prior art auto-focus cameras, the present in-focus state of the photographic optical system is maintained. Therefore, despite changes in the object distance that result when the camera moves during the exposure process no readjustment is made to the focal point.
When held by hand, the camera moves in various directions during the photographic operation. For example, during hand-held operation, the camera tends to move in both the pitching and yawing directions with respect to an optical axis of the camera. A hand-held camera also has a tendency to rotate in a rolling direction by rotating in the direction of the optical axis, or about the optical axis in a vertical or lateral direction. This rotation in the rolling direction results in parallel vibration, which tends to corrupt the in-focus state of the camera.
Snap shots account for a greater portion of photographic pictures that are taken. When taking a snap shot, camera shake tends to result in either a parallel or rolling direction. This parallel vibration generally does not affect the intended photographic image in terms of the object distance and the focal length, since the amount of movement of the subject that results on the image forming plane due to the parallel vibration is insignificant to compared to the amount of movement that occurs due to vibration in either the pitch or yaw direction. In addition, the effect of parallel vibration is also diminished since the effect of parallel vibration to the intended photographic image is diminutive compared with either the depth of the field, permissible shaking on the image forming plane, or the resolution of the human eye. Consequently, although compensation for the shaking of the photographic image in either the pitch or yaw direction is accomplished using a camera shake compensation mechanism, disclosed for example, in either of Japanese Laid-Open Patent Publication No. 4-18514 or No. 7-294975, no appropriate measures have been taken for out-of-focus images caused by other types of camera shakes.
For instance, in portrait photography, where the photographic range is expanded and the aperture is set to an almost open position to shoot the photographic subject in a close up view, minute changes in distance between a photographic subject and the camera have a significant effect on the photographic image. During close-up operation, the depth of the field is decreased, causing the image forming position to be out of the depth of field. As a result, a slight movement of the camera can cause an out-of-focus image. In this way, even if the focus has been set just prior to exposure, the intended photographic image may nonetheless be out-of-focus, since the object distance may momentarily change when the camera moves along the optical axis during a shutter release time lag, or during exposure.
A similar condition may also occur during close-up photography if the photographic image is taken by a camera that is held by hand rather than using a tripod. Since a photographic magnification level is particularly high in close-up photography, the depth of the field is small and becomes out-of-focus when there is a slight change in the object distance of the photographic subject, thereby considerably affecting the outcome of the resulting photograph.
As a result, the auto-focusing function of prior art cameras is not fully utilized since the auto-focusing function does not operate during the exposure process. As a result, prior art auto-focus cameras tend to produce out-of-focus photographic images when used for portrait or close-up photography.